1. Field of the Invention
This invention relates to golf balls and a method of manufacturing golf ball cores.
2. Description of the Related Art
Conventional methods of manufacturing golf balls of multiple-layer construction which avoid the golf balls from being eccentric are such as below:
1 First, an inner core is vulcanized and molded, then the inner core is held in a predetermined position in an outer layer metallic mold by, for example, a movable hold pin, and a material to form an outer layer is injected into the mold by an injection molding machine or a transfer mold. The hold pin is pulled out timely, and the inner core and the material injected into the mold are vulcanized. PA1 2 An outer layer, whose configuration is a hemispherical shell, is formed by semi-vulcanizing a material which is put between a hemispherical recessed mold and a hemispherical convex mold, or by heating a material which is put between a hemispherical recessed mold and a hemispherical convex mold for a fixed time so that the outer layer stays unvulcanized and is not shrank. Then the hemispherical convex mold is removed, and the outer layer shaped like a hemispherical shell is left on the hemispherical recessed mold, and an inner core that is vulcanization molded is set in the outer layer. And vulcanization press the inner core and the outer layer. PA1 3 This is the method 2 wherein a sheet-like outer layer is used instead of forming an outer layer shaped like a hemispherical shell.
The conventional methods described above are disclosed in Japanese Patent Provisional Publication No. 63-105774, Japanese Patent Provisional Publication No. 2-228978, and Japanese Patent Provisional Publication No. 6-218077.
However in the conventional methods of manufacturing golf balls, to reduce eccentricity of an inner core (inner layer), efficiency of manufacturing operation of balls was not concerned. For example, in the conventional method 1, the construction of the mold, which the inner core is held in the metallic mold by the movable hold pin, and the hold pin is pulled out as the material for forming the outer layer is injected into the mold by an injection molding machine or a transfer mold, is greatly complicated. The complicated construction of the mold costs high, and number of balls obtained by one press were small because of the limitation of pressure on injection, and the mold is not suitable for mass-production. Adjustment of clearance of mold for a movable pin is bothersome, because if the clearance is too small, the movable pin will not move well, and if the clearance is too large, rubber will flow out from the clearance.
In the conventional method 2, the problems raised in the method lD such as the smallness of the number of the golf balls obtained by one press, and the complicated construction of the metallic mold were solved. However when forming an outer layer to be a hemispherical shape, the formed hemispherical outer layer is needed to be left adhered to the hemispherical recessed mold for forming an outer layer, the most part of the outer layer was adhered to the hemispherical convex mold, and the operation of removing the outer layer to the hemispherical mold was required. Thus the method is not suitable for mass-production. In the conventional method 3, the outer layer which is a hemispherical shape in the method 2 is designed to be like a sheet. The problems raised in the method 2 are still left unsolved. Another conventional method besides 1 to 3 is disclosed in Japanese Patent Provisional Publication No. 61-25579. It is a known method that a projection of which the length is identical with the thickness of an outer layer is integrally molded on a surface of an inner layer, and the inner layer is covered with an outer layer of a quasi spherical shape shell. With the method, theoretically the eccentricity may be 0, however a projection is integrally molded on the inner layer and the easiness of releasing the layer from mold may be a problem. The inner core provided with a projection may be cut out from a material having a diameter which is same as the diameter of the outer layer, however it requires a long time, and the method is unrealistic.
It is therefore an object of the present invention to provide golf balls wherein the problems described above are solved, a remarkable reduction of eccentricity of an outer layer to an inner layer in a golf ball core is accomplished, and mass-production of the golf balls are possible. Moreover the golf balls superior in durability and their frying performance are objected to be obtained.
It is another object of the present invention to provide a method of manufacturing golf ball cores wherein an eccentricity of an outer layer to an inner layer in a core is remarkably reduced and mass-production of golf ball cores are possible.